Cellular and tumor immune responses were examined in animals injected with C. parvum or in tumor-bearing or tumor-immune mice. T lymphocytes from spleens of mice injected 14 days earlier with murine sarcoma virus (MSV) produce migration inhibition factor (MIF) when stimulated with intact tumor cells but T cells isolated from the tumor do not. Macrophages isolated from the tumor are capable of suppressing stimulation of MIF production by MSV immune spleen cells. Cytolytic macrophages can also be isolated from the tumor and represent a heterogeneous population of cells as noted by 1 g velocity sedimentation studies. Differences in subpopulations and levels of activity were observed in macrophages isolated from regressing and progressing tumors and from tumors in nude mice. Addition of endotoxin to the assay increased the levels of macrophage mediated cytotoxicity. Macrophage mediated cytostasis of tumor cells and lymphocytes could be demonstrated in both nude and conventional mice injected with either MSV or C. parvum. Suppression of lymphoproliferative responses by macrophages from tumor-bearing mice could be partially reversed when prostaglandin synthesis were inhibited. Macrophage tumor lines have been shown to exhibit cytostasis and cytolysis. Attempts have been made to establish long term T cell lines with cytolytic activity.